In general, when a moving object is photographed according to computed tomography, a motion artifact resulting from its motion appears in an image obtained by the tomography. The reason why such a motion artifact appears is that the computed tomography has a low temporal resolution with respect to the speed of the moving object. Therefore, the temporal resolution is heightened as follows. That is, (1) the rotational speed of a computed tomography apparatus (CT apparatus) is increased for a rapidly moving or vibrating organ, such as a heart, so as to have a rotation time of, for example, 0.5 seconds or less, or (2) an algorithm (half scanning) is performed to reduce the number of projection images by, for example, half, and these projection images are used when images corresponding to one rotation are reconstructed such that data acquired by one rotation is divided for each cardiac phase, and then pieces of data having the same cardiac phases are collected from data obtained through a plurality of scanning operations. These techniques make it possible to observe a time-dependent movement (i.e., a movement changing with the lapse of time).
On the other hand, a segment reconstruction algorithm is employed for a periodically moving or vibrating organ, such as a heart. According to this segment reconstruction algorithm, projection images that are different in the X-ray tube view angle of a computed tomographic image and that are the same in the expanding and contracting phase of the heart are collected and reconstructed. For this, if the heartbeat and the computed tomography rotation time (hereinafter, referred to as “CT rotation time”) coincide with each other, all X-ray tube view angles become equal to each other as shown in FIGS. 1(A) and 2 (i.e., 180 degrees in the example of FIG. 1(A), and 0 degrees and 180 degrees in the example of FIG. 2), and hence data for one rotation cannot be acquired, thus making it impossible to form an image. Therefore, various measures have been carried out. For example, the CT rotation time is changed so that the heartbeat of a subject, such as a patient, obtained from an electrocardiogram does not coincide with the CT rotation time as shown in FIGS. 1(B) and 3, and, as a result, synchronized projection images are not used (see Japanese Published Unexamined Patent Application Nos. H7-313504, H10-328175, and 2001-198121.)
In FIGS. 2 and 3, reference numeral 10 designates a patient, reference numeral 20 designates a gantry of a computed tomography apparatus (also called a “CT gantry”), reference numeral 22 designates an X-ray tube, and reference numeral 24 designates a scintillator arranged two-dimensionally.
However, conventionally, a transient movement, such as the flow of a contrast agent in a contrast examination, the movement of a catheter in a catheterization examination or a surgical operation, the movement of an insertion tube, or the movement of food and drink, was not able to be observed.
Japanese Published Unexamined Patent Application No. H9-75338, which is recognized as relating to this conventional technique, discloses a method in which, if the heartbeat is changed during photography of the heart, the rotation time is changed in accordance with a change in the heartbeat so as to improve the accuracy of ECG-gated segment reconstruction. Cardiac computed tomography is usually carried out while holding patient's breath for ten-odd seconds, and hence the heart rate inevitably becomes higher toward the last half of the tomography.
However, this method aims to obtain a three-dimensional image. Even if an image in which a cardiac phase has been changed is formed, there will be a difference in absolute time between CT slices. Therefore, it is impossible to see how a contrast agent or the like dyes.
Additionally, a cardiac muscle perfusion examination is performed according to a MRI method or a PET method, and has an important place in the cardiac examination.
If a cardiac muscle perfusion examination is performed according to the conventional computed tomography method, helical scanning or step & shoot scanning (which is repeatedly performed such that a patient is subjected to computed axial tomography at a position where the patient is lying, and is then subjected to axial tomography at a next position) is performed when ECG-gated or non-ECG-gated computed tomography is performed. Therefore, a time lag occurs between the photographing time of the apex of the heart and the photographing time of the bottom thereof. In addition, the influence of the heartbeat on image quality cannot be completely eliminated (i.e., an artifact does not disappear) even if ECG-gated computed tomography is performed, and hence the image quality is worsened, and the reliability of the result of a perfusion examination is decreased. Although the head of, for example, a patient is a motionless region, top priority is given to uniformity in the CT slice acquisition absolute time of a region undergoing a perfusion analysis in a head perfusion examination using the conventional computed tomography. As a result, the photographing range in the direction of a body axis becomes considerably narrow. As a result, it is impossible to diagnose the whole of diseased parts. In the worst case, it is known later that an important part has not been examined.
Concerning the conventional ECG-gated segment reconstruction and a technique relating to this, various algorithm or hardware attempts have been carried out to optimize the X-ray tube view angle of data acquired to form an ECG-gated segment reconstruction and heartbeat. Additionally, an attempt has been carried out to reduce a motion artifact and improve image quality.
On the other hand, the present invention aims to improve image quality in contrast while satisfactorily using a motion artifact even if temporal resolution is sacrificed.
For example, although a CT value of a diseased part is monitored and analyzed at time intervals in the perfusion examination, the fact that a motion artifact appears or disappears in response to the movement of an internal organ affects the CT value of the diseased part. This becomes a problem in perfusion analysis. In contrast with this, if a motion artifact constantly appears regardless of the movement of an internal organ as in the present invention, the CT value of the diseased part is not affected by its movement, and a wash-in & wash-out change in the contrast agent can be purely monitored.